1. Field of the Invention
The invention relates to crosslinked electrolytic film compositions useful in the preparation of separator membranes and hybrid electrolytes for electrolytic cells. For example, such a film material may be used in a rechargeable battery cell as an intermediate separator element containing an electrolyte solution through which ions from a source electrode material move between cell electrodes during the charge/discharge cycles of the cell. The invention is particularly useful for cells in which the ion source is lithium, a lithium compound, or a material capable of intercalating lithium ions. In that system, the film material may comprise a polymeric matrix which is ionically conductive by virtue of, for example, the incorporation of a dissociable lithium salt solution which can provide ionic mobility. One advantage of the present invention is the limited flow capability of the crosslinked material at elevated temperature enhancing the cell capability and decreasing the tendency to short circuit at high temperature, yet preserving its high ionic conductivity.
2. Background of the Invention
Early rechargeable lithium cells utilized lithium metal electrodes as the initial ion source in conjunction with positive electrodes comprising compounds capable of intercalating the lithium ions within their structure during discharge of the cell. Such cells relied, for the most part, on separator structures or membranes which physically contained a measure of fluid electrolyte, usually in the form of a solution of lithium salt, as well as providing a means for preventing destructive contact between the electrodes of the cell. Sheets or membranes ranging from glass fiber filter paper or cloth to microporous polyolefin film or nonwoven fabric have been saturated with solutions of a lithium salt, such as LiClO.sub.4, LiPF.sub.6 or LiBF.sub.4, in an organic solvent, e.g., propylene carbonate, diethoxyethane, or dimethyl carbonate, to form such separator elements. The fluid electrolyte bridge thus established between the electrodes has effectively provided the necessary Li.sup.+ ion mobility at conductivities in the range of about 10.sup.-3 S/cm.
Although serving well in this role of ion conductor, these separator elements unfortunately comprise sufficiently large solution-containing voids so that continuous avenues may be established between the electrodes, thereby enabling lithium dendrite formation during charging cycles which eventually leads to internal cell short-circuiting. U.S. Pat. No. 5,196,279 achieved some success in combatting this problem through the use of lithium-ion cells in which both electrodes comprise intercalation material, such as lithiated manganese oxide and carbon, thereby eliminating the lithium metal which promotes the deleterious dendrite growth. While providing efficient power sources, these lithium-ion cells cannot attain the capacity provided by lithium metal electrodes.
Another approach to controlling the dendrite problem has been the use of continuous films or bodies of polymeric materials which provide little or no continuous free path of low viscosity fluid in which the lithium dendrite may propagate. These materials may comprise polymers, e.g., poly(alkene oxide), which are enhanced in ionic conductivity by the incorporation of a salt, typically a lithium salt such as LiClO.sub.4, LiPF6 or the like. A range of practical ionic conductivity, i.e., over about 10.sup.-5 or 10.sup.-3 S/cm, was only attainable with these polymer composition at ambient conditions well above room temperature, however. U.S. Pat. No. 5,009,970 reported some improvement in mechanical properties while maintaining sufficient ionic conductivity of the more popular poly(ethylene oxide) composition by radiation induced cross-linking, while U.S. Pat. No. 5,041,346 reported improvement by meticulous blending with exotic ion solvating polymer compositions. Each of these attempts achieved limited success due to attendant expense, chemical instability and restricted implementation in commercial practice.
Some earlier examinations of poly(vinylidene fluoride) polymer and related fluorocarbon copolymers with trifluoroethylene or tetrafluoroethylene revealed enhancement of ionic conductivity by a simple incorporation of lithium salts and solvents compatible with both the polymer and the salt components. This work by Tsuchida et al. (Electochimica Acta, Vol. 28 (1983), No. 5, pp. 591-595 and No. 6, pp. 833-837) indicated, however, that the preferred poly(vinylidene fluoride) compositions were capable of exhibiting ionic conductivity above about 10.sup.-5 S/cm only at elevated temperatures reportedly due to the inability of the composition to remain homogeneous, i.e., free of deleterious salt and polymer crystallites, at or below room temperature. Such limitations apparently led to the abandonment of attempts to implement these compositions in practical rechargeable cells.
Prior work in U.S. Pat. No. 5,296,318 by the instant inventors developed a stable self-supporting film composition which is a copolymer of (poly)vinylidene fluoride with 8 to 25% by weight hexafluoropropylene (HFP). The material is self-limiting as below the 8% limit on HFP the crystallinity of (poly)vinylidene fluoride persists resulting in films incapable of sufficient uptake of the electrolyte solution, and above 25% HFP the formation of a self-supporting film may not be possible. These copolymers perform satisfactorily even after heating up to 70.degree. C.; however, the plasticized copolymer is soluble in the liquid electrolyte at temperatures higher than 80.degree.-95.degree. C. Melting of the electrolyte film under constant stress may result in the flow of the electrolyte and an internal shorting of the battery with the resulting fast discharge and heating.
It is well known that while the majority of fluoropolymers which do not contain specific functional groups undergo degradation by scission, rather than crosslinking when exposed to ionizing radiation, PVdF is an exception to the rule. It has been found in U.S. Pat. No. 3,142,629 that irradiation of PVdF to a dose of 8-50 Mrad with 4.5 MeV electron beam increases its mechanical properties at elevated temperatures by crosslinking the polymer. This patent discloses that only compositions containing more than 85% by weight of PVdF are suitable for e-beam crosslinking.
Elastomeric copolymers of vinylidene fluoride and hexafluoropropylene which are known under the Viton.RTM. (DuPont), FLUOREL.RTM. (3M), Technoflon.RTM. (Montecatini Edison), and DAI-EI.RTM. (Daikin) tradenames are chemically crosslinkable only at high temperatures (160.degree.-1900.degree. C.) in the presence of the diamine- or bisphenol-type additives. These additives could not be tolerated in the Li-ion battery due to side reactions with the active hydrogen atoms (--NH and -Ph-OH groups). It was thus surprising that films composed of poly(vinylidene fluoride) or its copolymer with hexafluoropropylene, crosslinked with dicinnamylidene hexanediamine (DIAK.RTM. curing agent, DuPont) in methyl ethyl ketone, and plasticized with a saturated solution of NH.sub.4 ClO.sub.4 in propylene carbonate were found by G. Feuillade and Ph. Perche, J. Appl. Electrochem. 1975, 5, 63-69, to be suitable separators for Li/FeS cells. In these systems, MgO or Ca(OH).sub.2 are added to the fluoropolymer elastomer as an HF scavenger. These additives, however, may be deleterious to the performance of the Li-ion cell.
The present invention provides a means for avoiding the disadvantages of prior rechargeable lithium battery compositions and constructions by enabling the ready and economical preparation of strong, flexible polymeric electrolyte material which function over a range extending well above and below room temperature.